A. Technical Field
The present invention relates to a water-absorbent resin having a treated surface and a process for producing the same. More particularly, the present invention relates to: a water-absorbent resin having a treated surface, which has a polyvalent metal atom on a surface of the water-absorbent resin; and a process for producing the same.
B. Background Art
As a sanitary material such as a disposable diaper, a sanitary napkin and an incontinent pad, a water-absorbent structure (also referred to as absorbent structure in some cases) containing a hydrophilic fiber such as a pulp and a water-absorbent resin as constituent materials is widely utilized for the purpose of absorbing a body fluid.
In recent years, high functionalization and thinning of these sanitary materials have progressed, and there is a tendency that the amount of a water-absorbent resin to be used per one sanitary material, and the ratio of the water-absorbent resin relative to the whole water-absorbent structure composed of the water-absorbent resin and the hydrophilic fiber are increased. That is, by reducing the hydrophilic fiber having a small bulk density, and using a large amount of a water-absorbent resin which is excellent in water-absorbing property and has a large bulk density, the ratio of the water-absorbent resin in the water-absorbent structure is enhanced, and the thinning of the sanitary material is tried without reducing the water absorption amount.
However, the sanitary material in which the ratio of the hydrophilic fiber is reduced, and the ratio of the water-absorbent resin is increased like this, is preferable from the viewpoint of simple liquid storage, but when distribution and diffusion of a liquid under circumstances of actual use as a diaper are considered, this is rather problematic. For example, the water-absorbent resin becomes a soft gel by much water absorption (moisture absorption), and there is a problem that gel blocking is caused such that diffusibility of a liquid in a sanitary material (liquid permeability of gel under load) and gel handling property are remarkably reduced. In order to avoid such the problem, and maintain absorption performance of the water-absorbent structure (also referred to as water-absorption performance in some cases), the ratio between the hydrophilic fiber and the water-absorbent resin is naturally limited, and a limitation is also generated in the thinning of the sanitary material.
Then, as means for preventing the aforementioned gel blocking, technique of adding a metal compound (in particular, polyvalent metal compound) to a water-absorbent resin is reported. Specifically, there is the technique of adding a powder of a metal compound (metal salt) such as aluminum sulfate to a water-absorbent resin, and then further adding water (e.g. see patent documents 1 and 2 below). However, since in such the technique, the added metal compound is dissolved and is permeated in the interior, even when the problem of the gel blocking is consequently overcome to thus obtain excellent liquid permeability and liquid diffusibility, a sufficient absorption capacity without load or a sufficient absorption capacity under load cannot be manifested, and there is a problem that desired absorption (water absorption) performance cannot be exerted.
Then, as means for overcoming such the problem, a technique of adding the aforementioned metal salt in the once dissolved state not in the form of the powder to the water-absorbent resin and, thereafter, drying to localize the salt near surfaces of the resin is reported (e.g. see patent document 3 below). In this technique, an attention is paid to a specified polyvalent metal atom, and the metal salt (metal compound) is used.
In addition, the water-absorbent resin exhibits a high absorption capacity when contacting with an aqueous liquid and, due to such water absorption property, the resin also absorbs a moisture in the air (moisture absorption), causing blocking phenomenon in which water-absorbent resin particles are aggregated, and flowability as a powder is lost. Regarding this blocking phenomenon (moisture absorption blocking phenomenon), a problem is known that, during the production of the water-absorbent resin, and during the production of the sanitary material such as a disposable diaper and the like using the water-absorbent resin, blocking and adhesion to apparatus are caused midway of a hopper and a line, so that the handling property is greatly deteriorated, and that the stable production cannot be performed. Thus, as a means for overcoming such the blocking phenomenon (moisture absorption blocking phenomenon), an attempt to add an inorganic compound to a water-absorbent resin is known.
For example, there are the following: a method of mixing an inorganic compound into a water-absorbent resin in which a ratio passing through a sieve of a mesh opening size of 300 μm is about 60% by weight (e.g. see patent document 4 below); a water-insoluble water-absorbent resin composition in which water has been imparted to a mixture of a polyvalent metal salt and a water-absorbent resin (e.g. see patent document 1 below); a production process of scattering water containing a polyvalent metal salt to a water-absorbent resin (e.g. see patent document 5 below); an improved water-absorbent resin in which water containing a polyvalent metal salt has been added to a surface of a water-absorbent resin having a particle size of 5 to 500 μm and this has been heat-treated (e.g. see patent document 6 below), an improved water-absorbent resin obtained by adding water with an inorganic salt dissolved therein to a water-absorbent resin having a crosslinked surface(e.g. see patent document 7 below), and a water-absorbing agent in which a polyvalent metal compound has been added to a water-absorbent resin, and a polyvalent metal is localized near a surface of the water-absorbing agent after water absorption (e.g. see patent document 3 below).                [Patent Document 1] JP-A-257235/1986 (Kokai)        [Patent Document 2] JP-A-523289/2001 (Kohyo)        [Patent Document 3] JP-A-096151/2001 (Kokai)        [Patent Document 4] JP-A-523526/2002 (Kohyo)        [Patent Document 5] JP-A-007745/1987 (Kokai)        [Patent Document 6] JP-A-046241/1986 (Kokai)        [Patent Document 7] JP-A-124879/1997 (Kokai)        
However, by such the technique, the problem of the gel blocking and the problem regarding the absorption capacity without load and the absorption capacity under load are overcome to a certain extent, but the liquid permeability under load cannot be said to be on a sufficient level, thus being problematic.